Machine and process controllers include a controller processor and I/O modules, the latter connecting to I/O devices on a machine or process. The term “I/O modules” is a general classification that includes input modules that receive signals from input devices such as photo-sensors and proximity switches, output modules that use output signals to energize relays or to start motors, and bidirectional I/O modules, such as motion control modules which can direct motion devices and receive position or speed feedback. Early I/O modules converted between AC and DC analog signals used by devices on a controlled machine or process and +5-volt DC logic signals used by the controller. Later I/O modules provided digital signals to digital I/O devices and received digital signals from digital I/O devices. Some I/O modules that are used to control motion devices or process control devices require local microcomputing capability on the I/O module.
Input data is collected from I/O modules and communicated to the controller processor. The controller processor performs logic operations on the input data to produce output data which is then communicated back to the I/O module having output capability. Controller processors have grown in computational ability and thus have increased communication requirements to larger groups of remotely located I/O devices. This has resulted in the common use of communication scanners and adapters. An I/O scanner is located near the controller processor and interfaces the controller processor through a distributed I/O network having a plurality of remote locations which can be at great distances from the controller processor. At various locations in the network, collections of I/O modules are interfaced in groups to the network by a communication adapter module known as an I/O adapter.
Originally one group of controller products was developed for machine and assembly line control, while another group of controller products was developed for process control. With the advances in microelectronics and microcomputers, the product lines are becoming suitable for both types of applications.
In process control, such as in the food and beverage industry, or in the petrochemical industry, there is a need for redundancy of systems to avoid an interruption in operation that would lead to a loss of the process batch. In machine and assembly line control, control systems must be designed so as to avoid down time. In many computer operations, there is a need for redundancy. Quite often this has led to complete redundancy of computers, using a primary computer and a backup computer operating in tandem.
Flood et al., U.S. Pat. No. 5,777,874, issued Jul. 7, 1998, disclosed a programmable controller backup system in which a primary controller processor was linked with a backup controller processor, each processor having an associated I/O scanner for communicating over a network to groups of I/O modules. If the primary controller processor became unavailable, control was shifted to the backup controller processor and its I/O scanner.
Flood, U.S. Pat. No. 5,912,814, issued Jul. 15, 1999, addressed a further problem in such a backup system in which the input data in the I/O table in the backup controller processor is not as current as the input data in the I/O table data in the primary controller processor. This can result in the output devices being set to a prior state at the time of changeover to the backup system and followed by a return to the present state and this is known as a “data bump.” The Flood '814 patent provides a solution for bumpless switching from the primary controller processor to the backup controller processor.
The provision of redundant controller systems is a solution with substantial cost in terms of equipment. It would be advantageous to provide redundancy in other ways that would be less costly and more directly related to the type of faults that may occur in controller systems.